In recent years, reflective type liquid crystal displays which use liquid crystal which exhibits a cholesteric phase at room temperature (typically, chiral nematic liquid crystal) have been studied and developed to be used as media for transforming digital information into visual information. This is because such liquid crystal displays have the advantages of consuming little electric power and of being fabricated at low cost. Such liquid crystal displays which use liquid crystal with a memory effect, however, have the disadvantage of having a low driving speed.
In order to solve this problem, the present applicants suggested, in Japanese Patent Application No. 2000-39521, an improved method of driving a liquid crystal display of this kind. By this driving method, it is possible to drive such liquid crystal by a low voltage and at a high speed.
According to the driving method, in order to display an image on such a liquid crystal display, the following steps must be carried out: a reset step of resetting liquid crystal to an initial state; a selection step of selecting the final state of the liquid crystal; an evolution step of causing the liquid crystal to evolve to the selected state; and a display step of displaying an image. Further, the selection step is composed of a selection pulse application step of applying an selection pulse, and a pre-selection step and a post-selection step which are respectively before and after the selection pulse application step.
Incidentally, chiral nematic liquid crystal has a characteristic that its responsibility to an electric field applied thereto is dependent on temperature. Accordingly, a liquid crystal display which uses chiral nematic liquid crystal may make an incomplete display or may not be able to make a display thereon depending on the temperature. In order to solve this problem, it was suggested that the waveforms of driving pulses be changed similarly during all the driving steps by changing the basic clock with changes in temperature (see SID98 DIGEST, pages 794–797).
The available temperature range in which such a liquid crystal display is used must be designed to be sufficiently wide, for example, from −20° C. to 60° C. If the basic clock is changed for temperature compensation within this wide range, the length of the selection pulse application step, which is the reference of scanning, changes largely, which results in too large changes in scanning speed.
Also, as the temperature becomes higher, the selection pulse application step becomes shorter, and in order to send image data to a signal electrode driving IC during this very short period, a high performance driver is required. Thus, the cost for the driver becomes high.
In the above-described temperature compensation method in which all the driving pulses are changed similarly, both the following problems must be solved: when the temperature is low, the writing speed becomes low; and when the temperature is high, high-speed data transmission is required. Also, when the temperature is high, that is, when the selection pulse application step is short, the wave of the selection pulse may be deformed because of the relationship between the resistance of the electrodes and the capacity of the liquid crystal, and necessary driving energy may not be applied to the liquid crystal.
An object of the present invention is to provide a liquid crystal display driving method which carries out temperature compensation while solving the both problems of the reduction in writing speed within a low temperature range and of the necessity of high-speed data transmission within a high temperature range, and a liquid crystal display apparatus which is driven by this method.
Another object of the present invention is to provide a liquid crystal display driving method which, in addition to attainment of the above object, inhibits the influence of deformation of the selection pulse so as to apply necessary energy to the liquid crystal within a high temperature range, and a liquid crystal display apparatus which is driven by this method.